A. Expanded Metal Sheets
Expanded metal sheets have found a variety of uses, from mats used for fighting fires to filters for automobile airbag inflators. They can be made in a variety of ways. For example, an expanded metal sheet can be made by taking a sheet of metal, puncturing the sheet to produce a multiplicity of slits, and pulling the sheet perpendicular to the direction of the slit to elongate the slit and provide an opening in the sheet. Another common method for making an expanded metal sheet is by piercing and cold forming openings, which are often called “diamonds” because of their final shape. The final length of the sheet, with the accompanying holes, is longer than the original and so it is expanded, as well as the openings formed being expanded.
Thus, although the details will vary depending on the specific process, expanded metal sheets are typically made by using a row of teeth or bits in a punch to produce perforations in the sheet. The side of the sheet facing the punch will have an indentation around the perforation, and the reverse side of the sheet will have a corresponding raised portion, a burr, around the perforation. The regularity of the perforations allows nesting of the perforations when the sheet is stacked, curled, rolled or otherwise placed in overlying relation, and the presence of burrs can lock the structure in a nested configuration. The burr accompanying each perforation also creates an area of increased friction so that the expanded metal sheet may not slide easily, and especially not when in contact with itself when curled or wrapped around a similar sheet.
B. Filters for Automotive Airbag Inflators
Filters for automotive airbag inflators need to satisfy a number of demanding criteria. Such filters serve to capture the extensive debris that is generated when an airbag's explosive charge ignites. This debris can damage the airbag and if released from the airbag can injure occupants of the vehicle in which the airbag deployed. In addition, the debris is often chemically harmful to humans.
To control this debris, filters for automotive airbag inflators need to be highly effective in their filtering function. Yet, they must also allow the gas generated by the explosive charge to quickly reach and inflate the airbag. That is, the filters cannot generate excessive levels of backpressure. Moreover, the filters need to satisfy these conflicting criteria, i.e., effective filtering with low backpressure, in the midst of a strong explosion. Besides these criteria, the filter also serves as a diffuser to attain a more even flow of the expanding gases entering the airbag and as a heat sink to help reduce the temperature of the gases so that they will not harm the airbag or the person being protected by the airbag.
In addition to these considerations, cost is always an issue for a mass-produced item, especially one used in the automotive field. Consequently, there have been continuing and extensive efforts to produce low cost and yet highly effective filters for airbag inflators.